In such a context as previously outlined, various solutions may make use of the well-known design of a “buck” converter (i.e., wherein a current is supplied to a load via an inductor), possibly without an output capacitor and/or with a constant-current control strategy, instead of a typical constant-voltage control strategy, whereby here by a “constant” current we mean an “average constant” current, i.e. a current which oscillates and is always included within two limit values, so that the average value in time is constant.
FIGS. 1 and 3 show various solutions that can be resorted to in order to achieve a control function of the above mentioned kind, and FIGS. 4 and 5 show various ways to drive a switch or an electronic switch, such as a mosfet.
In all Figures, load LS fed from the converter can include for instance a light source, for example a light source including one or several LEDs, possibly forming a so called “LED string”.
In such an application, it is possible to achieve an adjustment of the average brightness and/or of the average colour (if LEDs with different colour spectres are used) through short circuiting the whole string or part of it by static means, or else by a PWM (Pulse Width Modulation) technique. In this particular design, the converter is required to be adapted to maintain current regulation with good accuracy, in spite of the voltage variations brought about by the modulating circuit (dimming): see for example U.S. Pat. Nos. 4,743,897, 7,339,323 or US2007/0262724 A1.
In FIGS. 1 to 3, reference DA denotes in general an operational amplifier, typically structured as differential amplifier (in the case of FIG. 3, two such amplifiers are present, respectively DA1 and DA2), while references L, D and RS, possibly followed by other suffixes, indicate in general an inductor, a diode and a resistor.
When it is used as a derivative resistor or shunt, resistor RS can be connected in series with load LS, or else with one of the switches responsible for switching (i.e. an electronic switch including a mosfet or a diode).
Specifically, in the diagram of FIG. 1, shunt resistor RS is connected in series between output inductor L and load LS. The current on the load is detected throughout the switching period of differential amplifier DA, which detects the voltage across resistor RS and drives a control module C correspondingly. This in turn drives main switch M (for example a mosfet) adapted to modulate the power supply towards load LS.
The arrangement in FIG. 1 is a good solution in case of decreased or slow output voltage variations, taking into account the performance limitations of amplifier DA in terms of dv/dt. The arrangement of FIG. 1 may be prone to common mode errors, which can jeopardize overall performance and limit the width of output voltage.
In the diagram of FIG. 2, where elements or components identical or equivalent to parts or components already described are denoted by the same references, shunt resistor RS is connected to the return from load to ground. Once again, current is detected throughout the switching period. In this case, amplifier DA is ground referenced (and therefore there is no problem due to common mode errors), but load LS cannot be connected directly to ground, which may be a serious problem in such applications which require the use of several strings (multi-string), wherein it is paramount to have a common return.
As has already been stated, the diagram in FIG. 3 includes two amplifiers, the first of which, DA1, senses the voltage drop across a shunt resistor RS connected in the input line, while the second, DA2, senses the drop across a resistor RB inserted, for example, into a voltage divider RA, RB connected in parallel to load LS. The control action on switch M is therefore carried out as a function of the output signals of both amplifiers DA1 and DA2. In this case, current is sensed only during the on-time of electronic switch M, by using and input side shunt (i.e. resistor RS) connected in series to switch M. Common mode errors of amplifier DA1 are reduced by static operation at a known and constant voltage.
The lack of current sensing during the off-time of switch M requires resorting to a slightly different control technique, while considering the off-time as inversely proportional to the output voltage. This therefore requires voltage sensing via divider RA, RB, together with a programmable timer for the off-time. The achievable accuracy is limited because of the indirect current evaluation process.
Another aspect to be accounted for is the nature of the main switch, which can include a mosfet transistor.
Two choices are possible in this case, N-type or P-type.
N-type is faster, less expensive and less dissipative than P-type; furthermore, the gate charge is much lower. N-type, however, requires a gate voltage which is higher than source voltage, and therefore higher than input voltage, which is usually the highest voltage in the circuit.
This calls for some kind of voltage booster, adapted to consist of a charge pump circuit. Moreover, the mosfet source terminal is floating, so a floating driver is also needed.
A P-type mosfet uses a gate drive voltage which is lower than source, and the source terminal itself is connected to a stable point, which simplifies the operation of the driver.
As a reference, the diagram in FIG. 4, where once again references already used in the previous Figures denote identical or equivalent components (with the addition, in this case, of a capacitor CB and a further diode DB), shows the presence of a bootstrap circuit, which powers a driver Dr driving the gate of mosfet M (in this case of the N-type). The bootstrap circuit includes a diode DB and a capacitor CB, which are connected to the output of switch M. In this case, the auxiliary supply of driver Dr only operates when switch M is periodically switched, and therefore no static bias can be provided to the gate.
The diagram in FIG. 5 shows the use, as switch M, of a P-type mosfet; in this case, it is possible to supply driver Dr, driving the gate of mosfet M, via a dissipative current generator.